This invention relates to a method of compression moulding articles incorporating hollow inserts.
In compression moulding, a mass of uncured rubber (which term is used herein to include both natural and synthetic rubber materials) is enclosed within a mould under high pressure (e.g. around 7xc2xd tons per square inch) and at an elevated temperature (e.g. around 140xc2x0 C.) sufficient to cause curing of the rubber. As the pressure and temperature of the moulding material rises, the material becomes more fluid and flows to fill the mould cavity. Over a period of typically around ten minutes, the material becomes cured and re-solidifies, after which the mould can be opened and the moulded product removed.
It is common to want to mould hollow inserts into compression moulded articles. For example, sports shoes such as golf shoes or football boots may require receptacles in the shoe sole to secure replaceable ground-gripping elements such as cleats or studs. The cleat is usually secured in the hollow receptacle by a screw-threaded engagement or a bayonet-type connection. There are also many engineering components of moulded material having hollow inserts. Thus, a vehicle door seal may require nuts to be moulded in, or an engine mounting block may have a female-threaded component moulded in. Similar components are used extensively in the aircraft industry.
Compression moulding can be effected with steel or other metal inserts without difficulty. However, it is found that hollow plastics inserts suffer deleteriously during compression moulding from the combination of heat and pressure applied to them. The inserts become distorted and any screw thread or similar formations within the cavity of the insert are seriously damaged or even wholly destroyed. As a consequence, it has so far not been possible to incorporate hollow plastics inserts into compression moulded articles on a production basis.
According to the present invention, in a method of compression moulding an article from rubber moulding material incorporating at least one insert having a cavity, the moulding taking place in an enclosed mould, at an elevated pressure, the method comprises during moulding creating a pressure within the cavity of the insert which substantially balances the pressure exerted on the exterior of the insert by the rubber moulding material in the mould.
It has been found that pressurising the interior of the insert counteracts the crushing effects of the mould pressure applied to the exterior of the insert, so that damage to the insert is avoided.
Various techniques for pressurising the cavity of the insert could be employed. For example, pressure could be applied hydraulically by introducing oil. Alternatively, an inflatable insert-locating pin could be used within the cavity, the pin being expanded to fill the cavity and exert pressure.
However, a preferred technique is to introduce some of the rubber moulding material (or a material with suitably similar performance) into the cavity of the insert. This material itself becomes pressurised during the moulding process and so supports the insert internally to prevent damage from the external pressures.
Moulding material may be allowed, by provision of a suitable channel in the mould tooling, to feed into the cavity of the insert during moulding. However, this leaves a plug of material formed within the cavity, and secured by a linking piece to the rest of the moulded article. The linking piece then has to be separated from the article when the article is removed from the mould. It is, therefore, preferred that a suitable quantity of moulding material is introduced into the cavity of the insert separately, being introduced prior to the mould being closed and moulding commencing. As moulding proceeds, heat and pressure are transmitted to the material within the cavity of the insert, the material flowing to fill the cavity and protect the thread and/or other formations within the cavity. Preferably sufficient material is provided in the cavity of the insert to ensure that all parts of the cavity are protected by the material. It may also be sufficient to provide an excess which overflows into an overflow chamber provided in the tooling. The material in the cavity forms a plug, while that in the overflow chamber forms a handle which eases grasping of the plug for removal from the cavity after moulding.
Preferably each insert is supported during moulding on a locating means, including a pin for insertion into the cavity and a tube for location of the exterior of the insert. The pin may be part of the mould tooling. The moulding material introduced into the cavity may then be provided by a sheet of moulding material which is cut by the locating means as the insert is placed on the locating means. Alternatively, the pin may itself be formed from moulding material, to provide the necessary material in the cavity of the insert.
The invention is particularly useful for moulding shoe soles with inserts comprising socket-forming receptacles adapted to secure a ground-gripping element to the underside of the sole. Using the method of the invention, the receptacles may be pre-formed from plastics, and moulded into the shoe sole without being damaged.
There now follows a description, to be read with reference to the accompanying drawings, of a compression moulding process which illustrates the invention by way of example.